1. Field of the Invention
The present invention relates generally to liquid crystal display devices of the active matrix type and, more particularly, to a liquid crystal display device with avoidability of on-screen image display irregularities such as line defects occurring due to cutting failure and/or re-attachment of melt components during cut-away processes of electrode leads for use in driving picture elements or xe2x80x9cpixelsxe2x80x9d.
2. Description of the Related Art
In recent years, active-matrix liquid crystal display devices with thin-film transistors used as its switching elements are widely employed as those liquid crystal display devices adaptable for use as monitor units of various types in image visualization equipment and/or information processing apparatus.
A typical liquid crystal display device of this type is arranged to include an active matrix substrate with a plurality of (multiple) thin-film transistors for pixel selection formed on an electrically insulating or dielectric substrate and a color filter substrate having thereon a common electrode and color filter, which substrates are bonded together to define therebetween a thin gap space in which a layer of liquid crystals is sealed to thereby provide a liquid crystal panel with requisite driver circuitry assembled together.
While the liquid crystal display device of this type has traditionally been designed to employ so-called xe2x80x9ctape automated bonding (TAB)xe2x80x9d techniques for using a flexible printed circuit board tape carrier package (TCP) with built-in driver circuits mounted thereon to electrically connect between on-board electrical leads, such as image signal electrode leads and scanning electrode leads, and the driver circuits operatively associated therewith, the trend of modern LCD industry is to employ chip-on-glass (COG) architectures for directly mounting the driver circuits of a liquid crystal panel on one of the substrates thereof as the requirements are becoming more strict to further increase the effective display screen area while reducing the panel thickness when assembling into portable personal computers and the like.
These conventional liquid crystal display devices are designed to be equipped with an electrostatic protective lead (anti-static lead) for protecting the thin-film transistors or the like thereof from static damages during manufacturing processes of an active matrix substrate thereof. The electrostatic protective lead is electrically connected to an image signal electrode lead as provided inside of the liquid crystal panel at an extra portion of a mother board which will be removed from the liquid crystal panel during a cut-out process thereof from the mother board. The image signal electrode lead and electrostatic protective lead are formed of a thin film made typically of indium-tin-oxide (ITO), respectively.
FIGS. 8A and 8B are diagrams for explanation of an image signal electrode lead extension portion of a liquid crystal panel constituting one of conventional liquid crystal display devices, wherein FIG. 8A illustrates a plan view of main part corresponding to a single image signal electrode driving circuit whereas FIG. 8B depicts in cross-section an image signal electrode lead thereof.
As shown in FIG. 8A, this liquid crystal panel is structured from an active matrix substrate 1 and a color filter substrate 10 as bonded together to define a gap space therebetween in which a layer of liquid crystals is sealed. At the periphery of the active matrix substrate 1, a common lead 5 for electrostatic protection is formed outside of a cut line 7 to be cut during manufacturing. This electrostatic protective common lead 5 is formed with an electrostatic protective lead 4 while a plurality of image signal electrode leads 2 that are extended from the side of an effective display area AR are connected to this electrostatic protective lead 4.
Reference numeral xe2x80x9c3xe2x80x9d designates external connection terminals each serving as an input terminal of an image signal electrode driver circuit (IC) 6, to which an output of external circuitry, not shown, is coupled. A separation section 8 is formed between a respective one of the image signal electrode leads 2 and its associated electrostatic protective lead 4, wherein irradiation of laser light on a laser scan region 9 of this separator section 8 may enable cut-away for separation between the image signal electrode lead 2 and electrostatic protective lead 4 from each other.
As better shown in FIG. 8B, the separator section 8, which couples the image signal electrode leads 2 and the electrostatic protective lead 4, is comprised of an ITO layer 80 in the form of a transparent conductive film. The ITO separator section 8 extending between the image signal electrode leads and the electrostatic protective lead is cut by a laser beam before or after the active matrix substrate is cut into a product size, while letting the driver circuit (IC) 6 be mounted at this cut portion.
Unfortunately, in view of the fact that the ITO material is inherently high in electrical resistivity and also has a wide lead width splashy fragments or particles can badly behave to again attach or re-adhere to those portions between neighboring ones of the image signal electrode leads 2 during the cutaway processes using laser light, resulting in creation of undesired electrical short-circuiting therebetween. In addition, while a YAG laser that is widely used today to effect such a cutaway process of this kind measures 1.065 micrometers (xcexcm) in wavelength, ITO material exhibits a high transmissivity of those rays of light with wavelength values greater than or equal to that of visible light, so that the laser light cutting processes are less efficient. Due to this inefficiency, several ITO portions in the cutting area fail to be removed away and remain thereat, which sometimes results in a failure to adequately separate the image signal electrode leads 2 and electrostatic protective lead 4 from each other.
While swarf being blown up and reattached and/or remaining at cut portions causes electrical short circuits on the active matrix substrate, line defects take place on the display screen of the liquid crystal display panel, cause product defects thereof, and thus reduce the manufacturing yield and reliability thereof.
One of objects of the present invention is to avoid the aforementioned problems in the prior art and to provide a liquid crystal display device suitable for improving the manufacturing yield and product reliability thereof.
To attain the foregoing object, the present invention provides a specific device structure wherein a separator section for use in cutting away by laser light for separation of those electrode leads formed on an active matrix substratexe2x80x94specially, image signal electrode leads and electrostatic protective leadxe2x80x94is arranged to have a laminated structure including a low-resistivity metal layer and a high-melting-point metal layer (e.g. a refractory metal layer). In addition, this laminated structure is coated with an insulating film.
(1) One of typical structures of the liquid crystal display devices in accordance with the invention is defined as follows.
The liquid crystal display device comprises:
a color filter substrate having a common electrode and color filters of multiple colors on an insulating substrate;
an active-matrix substrate having a plurality of scan electrode leads formed on an insulating substrate, a plurality of image signal electrode leads formed to cross over or intersect the scan electrode leads, a plurality of thin-film transistors as two-dimensionally laid out to connect to the scan electrode leads and the image signal electrode leads for forming an effective display area, picture element electrodes connected to the thin film transistors respectively, more than one additional capacitance element connected to the picture element electrodes, a common electrode lead terminal for connection to the common electrode as formed on the color filter substrate, scan electrode lead terminals railed to extend from the scan electrode leads and image signal electrode leads up to one outer side of the effective display area along with image signal electrode lead terminals extending to a remaining side neighboring to the one side, and a dielectric protective film coating at least the thin film transistors, said active-matrix substrate opposing said color filter substrate with a specified gap space defined therebetween for letting liquid crystals reside therein;
driver circuitry including a scan electrode driver circuit and an image signal electrode driver circuit for supplying the scan electrode lead terminals and the image signal electrode lead terminals with a signal voltage for use in visually displaying images;
a separator section connected in common to the image signal.electrode lead terminals during manufacture of the active-matrix substrate for separating between an electrostatic protective lead for use in protecting said thin film transistors against static electricity and the picture element signal electrode lead terminals, wherein
at least one of separation sections for use in separating from the image signal electrode lead terminals the electrostatic protective lead for protecting from any possible static electricity the thin-film transistors connected in common to these image signal electrode lead terminals during manufacturing processes of the active matrix substrate has an arrangement including a multi-layer lamination structure of a low resistivity metal and a high-melting-point metal.
This separator section may be either part of the image (pixel) signal electrode lead terminal or part of the electrostatic protective lead. In either case, it will be desirable that the liquid crystal display device is manufactured in a way such that upon completion of a product those lead layers having the above-noted laminated (e.g. multi-layered) structure oppose each other between the image signal electrode lead terminals and electrostatic protective lead.
It should be noted that in the case of provision of an electrostatic protective lead on the side of the scan electrode leads also, letting a separator section of this scan electrode lead side and its associated electrostatic protective lead have a similar multi-layered structure makes it possible to obtain similar effects to those in separation of the image signal electrode leads.
(2) Said separator section in (1) is arranged so that it includes a layer made of chromium or an alloy containing therein chromium as its major component and a layer of aluminum or an alloy containing therein aluminum at an, amount of 50 percent or more plus a dielectric protective film, these layers being laminated over each other in this order of sequence as looking at from the side of said active matrix substrate (the principal surface side of this substrate).
With such an arrangement, in case the high-melting-point metal has a low-resistivity metal layer on the top surface thereof, when the high-melting-point metal is vaporized due to irradiation of laser light, the overlying metal layer will simultaneously be removed awayxe2x80x94or alternatively, where it has a dielectric film, this film will be removed awayxe2x80x94thereby absenting either electrical contact between neighboring electrode leads or contact with respect to the electrostatic protective lead, which results in improvements in production yield and product reliability.
Also note that the present invention should not be limited only to the above-noted arrangements and more than one embodiment as will be described later in the description and may be modified into a wide variety of forms for practical implementation of the inventive teachings as disclosed herein without departing from the spirit and scope of the invention.